NEW YORK — Pluto was once a planet. Then, a dwarf planet. And as of last week, a plutoid. The fall from grace has teachers, parents and educational publishers struggling to keep up, while kids remain loyal to their favorite, the ninth planet. Underscore planet.

Last week, the International Astronomical Union (IAU) announced Pluto should now be called a "plutoid," two years after the organization voted to demote Pluto to "dwarf planet" status.

Meanwhile, many kids are nearly certain Pluto is still a planet.

"I think it's a planet. But me and my friends, we talk about it sometimes and we go back and forth," said Natalie Browning, 9, sitting in a park in Manhattan with her family. "Right now, I'm not 100 percent. I'm just 75 percent" sure that Pluto is a planet.

Natalie's mom, Bobbie Browning, said, "You've got kids with textbooks saying that Pluto is part of the solar system and a planet, and teachers have to say it isn't [a planet]."

Science teachers and publishers already worked to update their resources to read "dwarf planet." And now, boom, that category is out of favor among astronomers.

"Students who have just learned about the concept of dwarf planets must now be taught the new concept of plutoid," said Janis Milman, who teaches earth science at Thomas Stone High School in Maryland. "This will lead to confusion in the classroom and resistance to learning the new terms, because the students will question, why learn something that might change again in a year or so?"

A cursory survey at a large chain bookstore here revealed three out of four books published in 2006 or later were updated, with Pluto designated as a dwarf planet and the solar system said to include just eight planets.

Chronicles of Pluto

Discovered in 1930 by Clyde W. Tombaugh at Lowell Observatory in Arizona, Pluto was always considered an oddball of sorts, with its tiny size (smaller than some moons) and eccentric orbit. During its 248-year trek around the sun, Pluto swings from its farthest point from the sun at 49.5 astronomical units (AU) to as close as 29 AU from the sun. One AU is the average distance between the Earth and sun, or about 93 million miles (150 million kilometers).

More than 70 years later, in August 2006, 424 astronomers at an IAU meeting voted to demote Pluto to dwarf planet status. Last week, the IAU Executive Committee reclassified Pluto as a plutoid. The other object in the plutoid club, Eris, is larger and more massive than Pluto.

Astronomers expect to find hundreds of Pluto-sized objects. And so the fate of Pluto will determine how these worlds are classified. For instance, new computer modeling suggests an object up to 70 percent of Earth's mass is lurking beyond Pluto. This "Planet X," if confirmed, would be called a plutoid under the IAU's scheme.

No matter what the scientists say, many kids won't let go.

"It's a planet," said fifth-grader Emily Mitchell, whose mother Laurie agreed, saying, "I grew up learning it was a planet."

"It's the smallest planet," said Liam, a 4-year-old who is "about to be 5." Liam's teacher Rachel Kaplan said, "I was really sad when Pluto was declassified as a planet, because I've studied astrology for a number of years."

Aileen Wilson said her 7-year-old son is interested in Pluto's label. "He's interested in why it was a planet and why it's not a planet anymore."

"I know that it was demoted and it's not a planet. But I don't know what it's called," said Erin Kelly, a pre-school teacher sitting on a park bench with her students in New York.

In the classroom

Even as scientists are arguing over the "plutoid" designation, with some saying they won't use the term, educators are already latching onto it.

Change is the name of the game in science, according to Gerry Wheeler, the executive director of the National Science Teachers Association.

"Basically, it's a teachable moment for science teachers, because it shows the dynamic nature of science," Wheeler told SPACE.com. He added the NSTA will spread news of the plutoid category to science teachers in the fall.

Elementary school science teacher Lucy Jensen agrees: "Pluto has made it interesting studying our planets this year." She teaches at Joliet Public School in Montana. "Our only problem we now have is buying new material, such as posters, videos, DVDs and game/study materials that need to be updated," she said.

Jensen added that while her fourth-grade students were more upset than the third graders about Pluto's demotion, the parents were the most upset. "It is hard to teach old dogs new tricks, and we like what we know," she said.

"Time has always been taken in the classroom to ponder the origin of Pluto. When Pluto became a dwarf planet, along with Eris and Ceres, it made it easier to explain why an object of Pluto's small stature could be classified," high-school teacher Milman said. "Now we will just need to teach them more new definitions."

Milman added that "dwarf planets" is an easier term for students to grasp compared with plutoids. "Objects of Pluto, Eris and Ceres' size are too small to be called planets so they were called dwarf planets. That was easier for the students to understand," she said.

Yet many students are still unaware of the change made in 2006.

"My fourth graders still consider Pluto a planet," said Bev Grueber, a science teacher at North Bend Elementary in Nebraska. "We do extensive oral reports on the planets to meet a state standard, and everyone jumps for joy when they get Pluto. Last year, I left Pluto out of the draw and they asked where it was, so they still consider it a planet regardless of what the space scientists tell us the definition of that planet is."

Aram Friedman, who founded Ansible Technologies Ltd. in New Jersey, travels to schools to teach about astronomy using a portable planetarium. In a typical fifth-grade class, he teaches students the features of the inner planets and the outer planets. Pluto, he says, doesn't fit into those categories. That makes sense to kids.

Publishing lag

Many science textbooks have only recently caught up with the dwarf planet concept.

For publisher McGraw Hill Education, the 2008 elementary and secondary school science textbooks describe Pluto as a dwarf planet.

Middle schools with the current Holt Science and Technology textbooks would see Pluto defined as a dwarf planet. McDougal Littell Science took a slightly different approach.

"We didn't say how many planets there were, so we didn't have to make a lot of changes. We explained, historically, that it had been classified as a planet when it was discovered," said Dan Rogers, vice president and director of Holt McDougal's science and health product development.

"One of the reasons we were cautious is because we thought the whole thing was unresolved and was going to change again," Rogers said. "We're in the process of developing a brand new program, a new set of books."

In "Traveler's Guide to the Solar System," an astronomy book published in 2007 for kids age 8 to 10, the author notes, "Earth is the third of nine planets (some say eight, some say ten, but nine is kind of traditional), orbiting our local star, the Sun."

Starry Night, astronomy software that includes educational resources, refers to Pluto as a dwarf planet, according to content director Pedro Braganca. (Starry Night is a division of Imaginova Corp., which also owns SPACE.com.)

And soon, educational publishers may need to re-update material. Word has it astronomers are vowing to pursue a reinstatement of Pluto as a planet.